One method of production of magnesium metal is by the silicothermic route, using the reaction: EQU 2CaO+2MgO+Si.fwdarw.2Mg+Ca.sub.2 SiO.sub.4
In one industrial application of this process, the Pidgeon process, a charge of briquetted powders of calcined dolomite (calcium-magnesium oxide) and silicon is placed in an elongate metal retort having an end connected to a vacuum source for application of vacuum to the interior of the tube. The tube is located inside a furnace and is heated to the reaction temperature, about 1200.degree. C., and heat is maintained during the endothermic reaction. Magnesium has a low partial pressure at the reaction temperature, so that vacuum must be applied to draw off vapour phase magnesium and cause the reaction to proceed. Upon encountering a cooler part of the tube, the magnesium condenses. At the end of the reaction, this accumulation of condensed magnesium is removed and remelted in a separate process to recover the same.
This prior art procedure possesses considerable problems, involving the difficulty of heat supply under vacuum conditions, the necessity to use highly skilled labour under very adverse conditions, the necessity to use vacuum in the reaction batch operation, and the limiting of the recovery of magnesium to about 75 to 80% of the theoretical.
There are other industrial processes based on the silicothermic reaction but these similarly possess significant drawbacks, including adverse working conditions and process economics.